1. Field of the Invention
The present invention relates to optoelectric articles and a process for producing the same.
2. Related Art Statement
Lithium niobate (LiNbO.sub.3) single crystal and lithium tantalate (LiTaO.sub.3) single crystal have been expected as materials for optoelectronics. It is known that a thin film of the lithium niobate single crystal is formed on a substrate made of the lithium niobate single crystal or the like by so-called liquid phase epitaxial process. For example, according to the description in "Appl. Phys. Letters", Vol. 26, No. 1 (1975), pp 8-10, a thin film of the lithium niobate single crystal is formed on a substrate of the lithium tantalate single crystal by the liquid phase epitaxial process. According to the description in "Mat. Res. Bull.", Vol. 10 (1975), pp 1373-1377, a thin film of the lithium niobate single crystal is formed on a substrate of the lithium niobate single crystal by the liquid phase epitaxial process. According to the description in "J. Appl. Phys.", Vol. 70, No. 5 (1991), pp 2536-2541, a thin film of the lithium niobate single crystal is formed on a substrate of lithium niobate single crystal doped with magnesium oxide by the liquid phase epitaxial process.
The film-forming method in the liquid phase epitaxial process will be explained. FIG. 1 is a graph schematically illustrating a temperature schedule for a melt in case of the liquid phase epitaxial process. FIG. 2 is a graph showing a solubility curve of a pseudo-binary system such as LiNbO.sub.3 -LiVO.sub.3. First, lithium niobate (a solute) and LiVO.sub.3 (a melting medium) are charged and mixed together. A saturation temperature corresponding to a melt having a charged composition is taken as "T.sub.0 ". While a temperature of the melt is held at T.sub.1 higher than the saturation temperature T.sub.0, lithium niobate and LiVO.sub.3 are uniformly melted. In FIG. 1, "A" corresponds to this molten state. Then, the melt is cooled to a supercooled state by lowering the temperature of the melt to a temperature T.sub.4 lower than the saturation temperature T.sub.0. In FIG. 1, "C" corresponds to this supercooled state. A substrate is contacted with the supercooled melt.
The crystallinity of such single crystal may be evaluated by the half-value width of the X-ray rocking curve. For example, according to the description in "J. Cryst. Growth" 132 (1993) page 48-60, a substrate made of a lithium niobate single crystal doped with magnesium oxide is used to produce a thin film of a lithium niobate single crystal having a half value width comparable to that of the substrate.